1. Field of the Invention
The invention relates to a video processing unit for processing a series of images to be displayed on a display device, wherein a particular image is displayed in a plurality of periods called sub-fields, each of the sub-fields having a respective weight corresponding with a respective intensity level generated in this sub-field.
The invention further relates to a display apparatus comprising such a video processing unit.
The invention further relates to a method of processing a series of images to be displayed on a display device, wherein a particular image is displayed in a plurality of periods called sub-fields, each of the sub-fields having a respective weight corresponding with a respective intensity level generated in this sub-field.
2. Description of the Related Art
U.S. Pat. No. 5,841,413 describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds with a pixel (picture element) of the image that is to be displayed on the panel. In the operation of the plasma display panel, three phases can be distinguished. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase, in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase, in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel emits light during this sustain phase. The three phases together are called a sub-field period or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on, is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time resulting in a particular illumination level of the activated cells. Typically, different sub-fields have a different duration of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32, respectively. By selecting the appropriate sub-fields in which a cell is switched on, 64 different intensity levels can be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, whereby a code word indicates the intensity level of a pixel in binary form.
As described above, the light emissions in the different sub-fields for a given pixel are integrated in the eye of the viewer resulting in a certain intensity level for that pixel. If a moving object is displayed, an artifact occurs in the image. In such a situation, the viewer tracks the moving object so that the eye moves between sub-fields. The integration of the light emissions is no longer perceived as one pixel of a certain intensity, but as light coming from different locations. Japanese Abstract JP 8211848 describes the application of motion estimation to compensate for motion between images. For a given sub-field, a pixel is moved from the original position to a corrected position on the basis of the motion vector and the delay for the particular sub-field. The pixel is typically moved to a sub-pixel position, i.e., the corrected position does not coincide with a location of another pixel. A problem is that the decision whether or not a cell is to be switched on in the particular sub-field is to be taken on pixel level. For a particular pixel, the cell is switched on or off and one cannot switch on the cell for a fraction to account for the fact that the corrected position does not fully coincide with this particular pixel.
It is an object of the invention to provide a video processing unit as described in the preamble with an improved reduction of motion artifacts. This object is achieved, according to the invention, in a video processing unit comprising:
first intensity calculating means for calculating for a particular pixel on the basis of motion estimation from the series of images a desired intensity level at a point in time corresponding with a current sub-field,
means for obtaining, from the motion estimation, a current motion vector for the particular pixel,
second intensity calculating means for calculating an accumulated intensity level integrated by a human observer along the current motion vector over a number of earlier processed sub-fields, if any,
third intensity calculating means for calculating a target intensity level to be generated in the current and one or more subsequent sub-fields, if any, on the basis of the accumulated intensity level and the desired intensity level, and
decision means for deciding whether the particular pixel is to be lit in the current sub-field on the basis of the target intensity level and the weight of the current sub-field.
By determining the level of intensity that is already realized for a particular pixel in earlier processed sub-fields, the video processing unit, according to the invention, can make a reliable and robust decision as to whether or not the particular pixel must be lit in the current sub-field. Any rounding errors in previous sub-fields can be taken into account by looking back and establishing in which sub-fields particular sub-fields have actually been lit. Among others, the invention is based on the insight that every sub-field provides a new chance to adapt the current word (given by a complete set of sub-fields at that time), such that the required intensity level is approached as close as possible.
An embodiment of the video processing unit according to the invention is by processing in the order of decreasing sub-field weights, the desired intensity level can easily be reached without running the risk of an overshoot in a certain sub-field which cannot be corrected in later processed sub-fields.
It is a further object of the invention to provide a method as described in the preamble with reduced motion artifacts. This object is achieved according to the invention in a method comprising the steps of:
calculating for a particular pixel on the basis of motion estimation from the series of images a desired intensity level at a point in time corresponding with a current sub-field,
obtaining from the motion estimation a current motion vector for the particular pixel,
calculating an accumulated intensity level integrated by a human observer along the current motion vector over a number of earlier processed sub-fields, if any,
calculating a target intensity level to be generated in the current and one or more subsequent sub-fields, if any, on the basis of the accumulated intensity level and the desired intensity level, and
deciding whether the particular pixel is to be ignited in the current sub-field on the basis of the target intensity level and the weight of the current sub-field.